The Dislocation Behaviour and GND Development in Nickel Based Superalloy During Creep
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The Dislocation Behaviour and GND Development in Nickel Based Superalloy during Creep Soran Birosca1, Gang Liu1, Rengen Ding2, Cathie Rae3, Jun Jiang4,5, Ben Britton5, Chris Deen6, Mark Whittaker1 1 Institute of Structural Materials, College of Engineering, Swansea University, Bay Campus, Swansea SA1 8EN, UK. 2 School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK. 3 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. 4 Department of Mechanical Engineering, Faculty of Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, UK 5 Department of Materials, South Kensington Campus, Imperial College London, London SW7 2AZ, UK 6 Rolls-Royce plc, PO Box 31, Derby, DE24 8BJ, UK. Abstract In the current study, dislocation activity and storage during creep deformation in a nickel based superalloy (Waspaloy) was investigated, focussing on the storage of geometrically necessary (GND) and statistically stored (SSD) dislocations. Two methods of GND density calculations were used, namely; EBSD Hough Transformation and HR-EBSD Cross Correlation based methods. The storage of dislocations, including SSDs, was investigated by means of TEM imaging. Here, the concept of GND accumulation in soft and hard grains and the effect of neighbouring grain orientation on total dislocation density was examined. Furthermore, the influence of applied stress (below and above Waspaloy yield stress) during creep on deformation micro-mechanism and dislocation density was studied. It was demonstrated that soft grains provided pure shear conditions at least on two octahedral (111) slips for easy dislocation movement reaching the grain boundary without significant geometrically necessary accumulation in the centre of the grain. Hence, the majority of the soft grains appeared to have minimum GND density in the centre of the grain with high GND accumulation in the vicinity of the grain boundaries. However, the values and width of accumulated GND depended on the surrounding grain orientations. Furthermore, it was shown that the hard grains were not favourably oriented for octahedral slip system activation leading to a grain rotation in order to activate any of the available slip systems. Eventually, (i) the hard grain resistance to deformation and (ii) neighbouring grain resistance for the hard grain reorientation caused high GND density on a number of octahedral (111) slip systems. The results also showed that during creep below the yield stress of Waspaloy (500 MPa/700C), the GND accumulation was relatively low due to insufficient microscopic stress level. However, the regions near grain boundaries showed high GND density. Whereas, in addition to the movement of pre-existing dislocations (SSD and GND) at higher mobility rate under 800 MPa/700C above yield creep condition, large numbers of dislocations were generated and moved toward the grain boundaries. This resulted in much higher GND density but narrower width of high intensity GND near the grain boundaries. It is concluded that although GND measurement by means of EBSD can provide a great insight of dislocation accumulation and its behaviour, it is critical however to consider SSD type which is also contributes to the strain hardening of the materials. Keywords: EBSD, Dislocation, GND, Nickel, Superalloy, Creep, TKD. 1. Introduction Waspaloy, a registered trademark of Pratt & Whitney Aircraft, is a precipitation hardened nickel based superalloy which was developed from the Nimonic series of alloys. Waspaloy has considerable strength and corrosion resistance at temperatures up to 870ºC Whittaker et al., 2017. As in other nickel based superalloys, Waspaloy is strengthened by a fine dispersion of γ′ particles, Ni3(Al,Ti), which has an ordered L12 structure and precipitates coherently in the nickel-rich FCC (γ) matrix. Waspaloy, a wrought superalloy, is a promising candidate material for use in superheater and reheater tubes as a substitute for 9-12% Cr steels in power plants [Viswanathan and Bakker 2001] in addition to its wide applications in gas turbine discs [Yao et al., 2013 and Penkall et at., 2003]. However, Waspaloy is subjected to creep deformation at relatively high temperatures in jet engine applications. Moreover, for large scale power plant applications creep damage is widely considered to be the most critical damage mechanism which determines the operational conditions. Thus, the creep deformation resistance of the alloys is a critical aspect for power generation and aerospace sectors and in particular, aerospace industry that aims for hotter jet engine production. Therefore, understanding creep deformation mechanism of Waspaloy under various static loading conditions and environments is of great industrial and academic research interest. Moreover, better understanding of the creep deformation mechanisms can guide towards improved mechanical properties, which can be achieved through the optimisation of thermomechanical processing routes. It is widely recognised that extrapolation methods currently employed for creep life predictions are not conclusive. It is also agreed that power law based techniques which are not linked to micro- mechanical behaviour are also at risk of failing to capture behavioural changes which show marked changes in material creep life [Whittaker et al., 2017]. The opportunity to design optimised creep resistant alloys, however is dependent on understanding the manifestation of creep damage within the microstructure of materials and particularly the localisation of creep strain in the vicinity of grain boundaries. Many recent attempts to provide a robust and reliable extrapolation method for creep data have focussed around the Wilshire equations [Whittaker et al., 2017]. The creep lifing methods have been based on power law type equations, in which the minimum creep rate (εṁ ) and the rupture time (tf) vary with stress () and temperature (T) according to the relationship: 푛 푀⁄푡푓 = εm = 퐴 휎 exp (− 푄푐⁄푅푇)…………………………………….. (1) where Qc is the activation energy for creep, R is the gas constant and M is the Monkman Grant constant. A and M are constants, T is the temperature and σ is the stress. The stress exponent (n) and the activation energy (Qc) are originally proposed as constants, however, they vary depending upon the applied stress and temperature. The dominant creep mechanism is then determined by comparing the experimentally measured values of (n) and (Qc) to those theoretically predicted. The reduction in stress exponent (n) is widely attributed to a change from diffusion-controlled dislocation processes to creep mechanisms related to vacancy diffusion, whereas the reduction in activation energy is associated with a change from Nabarro-Herring creep to Coble diffusional creep [Harrison and Homewood, 1994]. The most recent applications of the Wilshire equations have attempted to reconcile shifts in activation energy with micro-mechanical behaviour, with a focus on dislocation interactions either with strengthening precipitates, grain boundaries, or through forest hardening. Recently, interrupted creep tests performed on Waspaloy by Whittaker et al., 2017 and they reported that dislocation interactions were varied significantly above and below the yield stress. Above the yield stress forest hardening mechanisms dominated resistance to deformation since tertiary ’ particles, which are responsible for significant strengthening of nickel based superalloys, were easily cut by dislocations at the high stresses. The freedom of dislocations to glide through the tertiary ’ particles allows for exceptionally high dislocation densities which add to the precipitation hardening of the virgin material. Under these conditions, the apparent activation energy as calculated using the Wilshire equations, shows a relatively high value of 400k J/mol. However, at stresses below the yield point (~ 730 MPa) cutting of tertiary ’ particles are reduced and the dislocation density falls [Whittaker et al., 2017]. They also observed a reduction in activation energy to (340 kJ/mol) as dislocation motion is less inhibited where the materials crept below yield stress. Hence, the difficulties have consistently arisen in fitting the constants for the Wilshire equations to the creep data sets of Waspaloy. Datasets can be fitted more accurately to two distinct sets of parameters in terms of minimum creep rate or rupture time with the ‘break points’ being a function of stress. A break point is commonly found to occur at stresses approximately equal to the yield stress of the material, and this is assumed to be related to a change in the dislocation behaviour at this point [Whittaker et al., 2017]. Therefore, the dislocation interactions must play a significant role in the determination of this value, and hence the behaviour of the material during creep. The current investigation seeks to evaluate the local creep strain accumulation below and above yield stress in Waspaloy in order to understand the exact features and micro/nano structural changes during creep that are responsible of the Wilshire equation data fitting deviations for better creep life prediction. For the past 50 years, numerous theoretical, experimental and computer simulation studies have been conducted on creep deformation of nickel based superalloys. However, most of the studies in literature have been based on dislocation based theories to understand the development, generation, movement, accumulation and mobility of dislocations during loading.